(1) Field of the Invention
The present invention is generally related to verification and calibration of color as well as corrective adjustments for an imaging device.
(2) Background Information
Colors are part of our daily lives. However, for most people, there is no unit of measurement for color like there is for length or weight. For example, when we say that grass is green, dark green or light green, it is a matter of perception and subject to interpretation. In other words, people will draw different conclusions based on their experiences and use different words to express color. Therefore, the results can only be vague when describing a color using such terms as described above. Likewise, similar problems exist when verifying and calibrating color as detected by an imaging device based on visual perception.
One method to obtain precise color description would be to employ highly trained inspectors to control color on a production line and visually evaluate colors generated by the imaging device to determine if the colors are within an acceptable range. However, the problem with this approach is that the visual inspection is not based upon a set of objective criteria, but is based upon the experience of the inspector, which is subjective. Furthermore, this type of work is limited to those who have acquired years of experience to develop the visual skill. Another problem is that the evaluation may vary between inspectors because the perception of colors vary with age. Moreover, inspectors, being human, are subject to fatigue, which limits the number of inspections that could be done per inspector.
A better approach is to quantify colors such that colors are expressed numerically and thus could be analyzed by a machine with a high degree of accuracy. Many methods have been developed to quantify colors. Two of the methods are those developed by an international organization known as Commission Internationale de l'Eclairge (CIE). The first method is known as XYZ tristimulus valves and its associated Yxy color space. Color space is a term defining a method of numerically expressing a color of an object. XYZ tristimulus values are based on a concept that human vision perceives color by mixing three primary colors: red, green, and blue. XYZ tristimulus values are charted in three-dimensional space. Y.sub.xy color space is an aid to visualize the color defined by the tristimulus values XYZ by graphing the color in two-dimensional space.
The other method is known as L*a*b color space. Under L*a*b color space, L indicates lightness, while a and b indicates the chromaticity coordinates of a color in three-dimensional space. Stated differently, a and b indicate color directions, i.e. +a is the red direction, -a is the green direction, +b is the yellow direction, and -b is the blue direction. When L*a*b values for a color has been assigned, that color is numerically specific under the L*a*b color space.
Yet another method is the Munsell system. The Munsell system expresses color by using numerous color chips which are classified according to the color's hue, lightness, and saturation. Hue is the classification of shade of a color such as yellow, orange, red, purple, blue, and etc. Lightness is the classification of whether the color is dark or light. Saturation is the classification of whether the color is bright or dull.
A known prior art technique which has been utilized numerically expresses color to verify and calibrate color in charge coupled devices (CCDs). There, several color sheets with known numerical color values using the Munsell system are placed in front of a CCD device to be tested. The image of the color sheet is then taken by the CCD device and processed by a computer to determine if the color detected by the CCD device is within a predetermined tolerance level in comparison with the known color value placed in front of the CCD device. However, this approach is time-consuming since an image is taken for each individual sheet. Moreover, time is further consumed because other tests such as contrasts and signal to noise ratios are performed separately for color verification and calibration. In a production line, as an example, such time-consuming activities amount to unacceptable productivity loss. Accordingly, what is needed is a method of verifying and calibrating several colors simultaneously while providing a variety of additional tests at the same time.